Lithium cells, particularly lithium secondary cells, since they have characteristics of high energy density, long life and the like, are used as power supplies for household appliances such as video cameras, and portable electronic devices such as laptop computers and cellular phones. Recently, the lithium secondary cells have been applied also to large-size cells mounted on electric vehicles (EVs), hybrid electric vehicles (HEVs) and the like.
Lithium secondary cells are secondary cells having a following structure. In which in the charge time, lithium dissolves out as ions from a positive electrode and migrates to a negative electrode and is intercalated therein; On the other hand in the discharge time, lithium ions reversely return from the negative electrode to the positive electrode, and their high energy density is known to be due to potentials of their positive electrode materials.
As positive electrode active materials of lithium secondary cells, there are known, in addition to lithium manganese oxide (LiMn2O4) having a spinel structure, lithium metal composite oxides having a layer crystal structure, such as LiCoO2, LiNiO2 and LiMnO2. Since for example, LiCoO2 has a layer crystal structure in which a lithium atom layer and a cobalt atom layer are alternately stacked through an oxygen atom layer, and is large in charge and discharge capacity and excellent in diffusability of lithium ion intercalation and deintercalation, many of lithium secondary cells commercially available at present employ lithium metal composite oxides having a layer crystal structure, such as LiCoO2 as positive electrode active materials.
Lithium metal composite oxides having a layer crystal structure, such as LiCoO2 and LiNiO2, are represented by the general formula: LiMO2 (M: transition metal). The crystal structure of these lithium metal composite oxides having a layer crystal structure is assigned to a space group R-3m (“-” is usually attached on the upper part of “3,” indicating rotatory inversion. The same applies hereinafter); and their Li ions, Me ions and oxide ions occupy the 3a site, the 3b site and the 6c site, respectively. Then, these lithium metal composite oxides are known to assume a layer crystal structure in which a layer (Li layer) composed of Li ions and a layer (Me layer) composed of Me ions are alternately stacked through an O layer composed of oxide ions.
The case where such a lithium metal composite oxide having a layer crystal structure is used as a positive electrode active material of a lithium secondary cell poses such a problem that since especially when the cell is charged and discharged at a high temperature, the lithium metal composite oxide chemically reacts with an electrolyte solution and changes occur including adhesion of a reaction product of the reaction on a surface of the positive electrode active material, the capacity and the charge-discharge cycle ability of the cell decrease.
As one example of means of solving such a problem, it is considered that the particle surface of a lithium metal composite oxide having a layer crystal structure is coated with a metal or a metal oxide.
For example, Japanese Patent Laid-Open No. 2001-291518 discloses a positive electrode active material for a lithium secondary cell in which the surface of a lithium metal composite oxide having a layer crystal structure contains an oxide or composite metal oxide layer of a metal(s) selected from the group consisting of Mg, Al, Co, K, Na, Ca, Si, Ti and V.
Japanese Patent Laid-Open No. 2005-310744 discloses a positive electrode active material obtained by coating with aluminum the surface of a particle obtained by dispersing and stirring a particle powder of a lithium metal composite oxide having a layer crystal structure in an isopropyl alcohol solution and thereafter subjecting the dispersion to a heat treatment at 600° C.
Japanese Patent Laid-Open No. 2005-322616 discloses a lithium-containing oxide in which the surface of an oxide is coated with a layer including aluminum hydroxide, aluminum oxide and lithium carbonate, the oxide being obtained by adding a lithium metal composite oxide having a layer crystal structure and a powdery metallic aluminum to water to make a slurry, further stirring the slurry to dissolve the metallic aluminum, and thereafter drying the slurry at 80° C.
Japanese Patent Laid-Open. No. 2005-346956 discloses a lithium-containing oxide in which its particle surface is modified with an aluminum compound, wherein the lithium-containing oxide is obtained by adding aluminum stearate to a lithium metal composite oxide having a layer crystal structure, mixing and crushing the mixture by a ball mill, and subjecting the resultant to a heat treatment at 600° C.
WO2007/142275 discloses a positive electrode active material for a nonaqueous electrolyte secondary cell, as the positive electrode active material in which a lithium metal composite oxide particle having a layer crystal structure is surface-modified so that aluminum in a specific, relatively high concentration is incorporated into its specific surface region, wherein the positive electrode active material is composed of a surface-modified lithium-containing oxide particle in which aluminum is incorporated into the surface layer of the lithium metal composite oxide particle having a layer crystal structure, and the aluminum content within 5 nm of the surface layer is 0.8 or higher in atomic ratio with respect to the total of Ni and elements M.
Japanese Patent Laid-Open No. 2008-153017 discloses, from the viewpoint of use for a positive electrode active material in which the surface of a lithium oxide having a specific composition and having a specific particle diameter and particle size distribution is coated, the positive electrode active material for a nonaqueous electrolyte secondary cell, which is a lithium metal composite oxide having a layer crystal structure and has a structure in which the surface of a lithium oxide for a nonaqueous electrolyte secondary cell composed of particles having a particle distribution of an average particle diameter D50 of 3 to 15 μm, a minimum particle diameter of 0.5 μm or larger, a maximum particle diameter of 50 μm or smaller, and a D10/D50 of 0.60 to 0.90 and a D10/D90 of 0.30 to 0.70 is coated with a substance A (A is a compound composed of at least one element selected from the group consisting of Ti, Sn, Mg, Zr, Al, Nb and Zn).
As described before, when the particle surface of the lithium metal composite oxide is coated with a metal or a metal oxide in order to suppress the reaction of the lithium metal composite oxide with an electrolyte solution, there resultantly arises such a new problem that the rate capability and the output characteristics of a cell worsen.